Atom transfer radical polymerization (ATRP) and reversible addition-fragmentation transfer (RAFT) polymerization of N-methyl methacrylamide and methyl methacrylate were investigated in the presence of rare-earth triflates known to enhance polymer isotacticity. Poly(N-methyl methacrylamide) with controlled molecular weight, low polydispersity and enhanced isotacticity was prepared by ATRP and RAFT in the presence of catalytic amounts of yttrium trifluoromethanesulfonate or ytterbium trifluoromethanesulfonate. The tacticity of poly(N-methyl methacrylamide) depends on the Lewis acid concentration well-defined polymers with predominantly either syndiotactic, tactic, or isotactic triads were prepared by adjusting the concentration of the Lewis acid. Simultaneous control of molecular weights, polydispersities, and tacticities in the polymerization of methyl methacrylate was less successful.